Vacuum process chambers are often employed in manufacturing to provide a vacuum environment for tasks such as semiconductor wafer fabrication, electron microscopy, gas chromatography, and others. Such chambers are typically achieved by attaching a vacuum pump to the vacuum process chamber in a sealed arrangement. The vacuum pump operates to remove substantially all of the molecules from the vacuum process chamber, therefore creating a vacuum environment.
One type of vacuum pump is a cryopump, such as that disclosed in U.S. Pat. No. 5,862,671, issued Jan. 26, 1999, assigned to the assignee of the present application and incorporated by reference in its entirety. Cryopumps remove molecules from a vacuum process chamber by cooling a surface to temperatures approaching absolute zero. At such temperatures, most all gases condense on the cooled surface, called a cryogenic array, thereby removing substantially all molecules from the vacuum process chamber.
Cryopumps typically employ a helium driven refrigerator to achieve the near absolute zero temperatures required. A compressor is used to compress and pump the helium refrigerant to the cryogenic refrigerator in the cryopump, and a cylindrical shaped vessel called a cold finger in the cryogenic refrigerator receives the helium. A cryogenic array is attached to and in thermal communication with the cold finger and cooled therewith. A displacer reciprocates inside the cold finger as the helium expands, driven by a displacer drive motor which reciprocates the displacer and regulates the quantity of helium used. As the helium expands in the cold finger, heat is drawn off the cryogenic array, generating the near absolute zero temperatures required to condense gases on the cryogenic array.
The amount of helium refrigerant available to the cryogenic refrigerator determines the rate at which cooling occurs. A greater supply of helium decreases the amount of time required for cooldown, which is the time required to achieve cryopumping temperatures. The rate of helium consumption also varies with the temperature of the cryogenic refrigerator. As the cryogenic refrigerator becomes colder, a greater supply of helium is required to continue the cooling process. In a cryopumped vacuum process chamber, downtime can result in substantial economic impact, due to lost manufacturing time. Accordingly, the capability to rapidly achieve and maintain cryopumping temperatures is beneficial.
One prior art type of helium distribution is described in U.S. Pat. No. 5,775,109, entitled “Enhanced Cooldown of Multiple Cryogenic Refrigerators Supplied by a Common Compressor,” filed Jan. 2, 1997 and assigned to the assignee of the present application, incorporated herein by reference in its entirety. This patent suggests individually monitoring the temperature of each of a plurality of cryopumps to control the speed of each displacer drive motor when a cryopump attains a triggering temperature. As cryopumps require varying amounts of helium depending upon the operation currently being performed, regulating the drive motor speed can reduce or increase the helium supply accordingly. In this system, each cryopump monitors temperature and controls the drive motor speed accordingly.
Frequently, however, a common helium supply manifold supplying a plurality of cryopumps is capable of supplying more helium than required by all of the cryopumps. Excess helium which is not identified is often unutilized, which can increase the time required for cooldown and which can cause a cryogenic refrigerator to become colder than needed, wasting power and other resources required to maintain the helium refrigerant supply.